Economical operation of high severity hydrocarbon cracking processes and equipment requires overcoming numerous competing operational and engineering challenges. The high temperatures and process stresses can exceed the long term viability of most conventional apparatus, including conventional refractory ceramics. In addition to component physical and thermal performance considerations, component chemical inertness and crystalline stability also become significant considerations. Degradation and corrosion present extra obstacles requiring address, particularly in temperature hydrocarbon processing.
One problem pertains to ceramic stabilizer volatility and progressive loss of such stabilizer from a ceramic matrix due to the severe temperatures and cyclic temperature swings. Related issues include carbon infiltration and coking within the ceramic matrix pores and an associated, undesirable carbide-oxide interaction chemistry resulting in progressive corrosion and degradation of the ceramic matrix. Such problems are of particular interest in high severity pyrolysis of hydrocarbon feedstocks (e.g., >1500° C.).
Conventional steam crackers are a common tool for cracking volatile hydrocarbons, such as ethane, propane, naphtha, and gas oil. Other higher severity thermal or pyrolysis reactors are also useful for cracking hydrocarbons and/or executing thermal processes, including some processes conducted at temperatures higher than can suitably be performed in conventional steam crackers. Compared to conventional cracking equipment and processes, higher temperature reactions and processes typically require more complex, costly, and specialized equipment to tolerate the intense heat and physical stress conditions. Properties such as temperature, reaction environment, component strength, and toughness limitations commonly define upper limits for many processes.
In addition to processes utilizing high temperatures (e.g., >1500° C.) posing equipment challenges, processes involving high temperature plus large cyclic temperature swings and directional changes of cyclic process fluid, such as in regenerative or reverse flow reactor processes, pose even greater challenges. For example, the art discloses that to efficiently obtain relatively high yields of acetylene from pyrolyzing methane feed, such as in excess of 75 wt. % yield, reactor temperatures in excess of 1500° C. are required and preferably in excess of 1600° C., with relatively short contact times (generally <0.1 seconds). Due to the high temperatures involved, such processes are generally limited to relatively small amounts or batches using cyclical processes yielding a mixture of acetylene, CO, and H2. Due to the high severity, such methane cracking processes, however, have been relatively inefficient, impractical, and of very limited commercial value as compared to other more economical processes for generation of acetylene. Acetylene is typically generated commercially by cracking feeds other than methane at lower temperatures.
The high temperature processes (e.g., >1500° C.) have previously not scaled well and are generally only useful for relatively high-cost, specialty applications. Processes such as thermally cracking methane to acetylene have been commercially unattractive due in large part to thermal, chemical, and mechanical degradation of the reactor equipment, including ceramic material used therein. Cyclic temperature changes and product flow direction changes impose severe physical strength and toughness demands upon the refractory materials at high temperature. Such stresses and performance demands have also typically limited manufacturing and use of the refractory materials to relatively simple shapes and components, such as bricks, tiles, spheres, and similar simple monoliths. Reactor component functions and shapes for all refractory materials suffer limited use in high severity services.
In addition to physical temperature limitations for reactor materials, many prior art ceramic reactor materials that are relatively inert at lower temperatures become susceptible to chemical degradation, ceramic corrosion, and/or crystalline alteration at higher temperatures, leading to premature degradation and/or process interference such as by generation of unacceptable levels of contaminants in the process. Although high temperature regenerative pyrolysis reactors are generally known in the art as capable of converting or cracking hydrocarbons, they have not achieved widespread commercial use, due significantly to the fact that they have not been successfully scaled to a commercially economical size or useful life span as compared to less severe alternatives, such as steam cracking.
The identified prior art pertaining to refractory materials for high-severity hydrocarbon pyrolysis dates primarily to the 1960's and earlier. However, that art merely occasionally provides generalized lists of some exemplary materials such as ceramics, alumina, silicon carbide, and zircon as reactor materials. These sparse, non-specific disclosures left the art largely incapable of providing a large-scale, commercially useful reactor or reactor process. The teachings of the art were only effective for enabling relatively small scale specialty applications that see vastly inferior use as compared to large scale processes such as hydrocarbon steam cracking. The identified art is void of teaching or providing a refractory ceramic material capable of sustaining the complex set of properties that are required for extended use in the reactive or other most-demanding regions of a high-severity (≧1500° C.) pyrolysis reactor, such as for the commercial production of acetylene and/or olefins from methane or other hydrocarbon feed. The studied art does not teach preferred crystalline structure or composition for particular reactor furnace uses, or for complex reactor component shapes and/or functions. Multimodal ceramics are also known in the ceramics art, as are ceramic compositions utilizing nanoparticles. However, the art remains void of teaching a ceramic or other composition or method of preparing the same that meets the rigorous performance properties needed for commercial application and long-term stability in high temperature cyclic pyrolysis processes. Further, the desired materials must maintain their formulations, crystalline structure, and corresponding physical and chemical properties for prolonged periods of time, at commercial scale and within the confines of an economic requirement. The studied art is believed similarly deficient at teaching such refractory materials, particularly those suitable for use as complex, irregular, relatively fragile, and/or functionally-shaped reactor components.
For further example, the “Wulff” process represents one of the more preferred commercial processes for generation of acetylene. Wulff discloses a cyclic, regenerative furnace, preferably including stacks of Hasche tiles (see U.S. Pat. No. 2,319,679) as the heat exchange medium. However, such materials have demonstrated insufficient strength, toughness, and/or chemical inertness, and are not amenable to use as certain desirable reactor components, such as for use as reactor fluid conduits, to facilitate large-scale commercialization. Although some of the “Wulff” art disclose use of various refractory materials, a commercially useful process for methane cracking or other extreme high-temperature processes has not previously been achieved utilizing such materials. The aforementioned practical obstacles have impeded large scale implementation of the technologies. Materials availability for high temperature, high-stress applications is one of the most critical issues in design and operation of large-scale, commercial, high-productivity, thermal reactors. Due to high temperatures involved in cyclic pyrolysis reactors, generally only ceramic components have the potential to meet the materials characteristics needed in such aggressive applications.
One attempt to overcome the aforementioned problems involved use of a “deferred combustion” process that delayed combustion and heat generation until the reaction components were positioned into the core of the reactor, thermally isolated from flow control equipment that was subject to premature degradation. The deferred combustion, regenerative reactor process and equipment was disclosed in a U.S. patent application filed Dec. 21, 2006, Sr. No. 11/643,541, entitled “Methane Conversion to Higher Hydrocarbons,” related primarily to methane feedstocks for pyrolysis systems. Although the disclosed process of the '541 application effectively controls the location of combustion within the reactor, the internal reactor components must still contend with the severely high temperatures, temperature changes, and physical stresses incurred during methane pyrolysis, particularly for a commercially desirable reactor life term. The refractory material comprising the reactive regions may typically be a ceramic or related refractory material. In some embodiments, however, the disclosed processes and material may utilize relatively complex shaped refractory components, such as a thin-walled honeycomb monolith used to conduct process fluids through the reactor. Such reactors and reactor component geometries demand materials that have strength, toughness, chemical inertness, and other required properties that exceed the capabilities of previously identified or known refractory materials under such temperature and stress conditions.
While many ceramics tend to be somewhat inert or chemically stable at moderately elevated temperatures, many ceramics become chemically and/or structurally unstable at severely elevated temperatures, tending to degrade and corrode within undesirably short periods of time, rendering them unsuitable for some applications. Exemplary chemically and/or thermally unstable ceramics include certain silicas, aluminas, borides, carbides, and nitrides. Many of such ceramics are also known to undergo alterations in crystalline structure at elevated temperatures and/or across relevant process temperature ranges. Such alterations can result in changes in bulk volume which result in creation of stress fractures and/or cleavage planes which in turn may reduce the material's strength or performance properties.
Zirconia is a crystalline material that is commonly used in certain refractory ceramics. However, zirconia undergoes a crystalline change between moderately high temperatures and severely high temperatures in the way its atoms are stacked (polymorphic transformation). Zirconia has a monoclinic crystal structure between room temperature and about 1200° C. Above about 1200° C., zirconia converts to a tetragonal crystal structure. At a still higher temperature, such as above 2370° C., zirconia changes from tetragonal to cubic structure and melts at 2715° C. These transformations are accompanied by volumetric shrinkage and expansion between the crystalline states, resulting in fractures or cleavages along grain boundaries. In polycrystalline zirconia, this tetragonal-monoclinic transition and cleaving results in a progressing reduction in strength and potential catastrophic failure of the component. Stabilizers, such as yttria (Y2O3) and some metal oxides can be incorporated within the crystal structure to arrest or prevent the crystalline shifts, rending the crystal structure more stable across a broader temperature spectrum.
A related problem has to do with stabilizer loss during high temperature (at least 1500° C.) hydrocarbon pyrolysis. It has recently been learned that extended exposure of stabilized ceramic components, such as but not limited to stabilized zirconias, to high temperature processes and severe environments can result in progressive evacuation or loss of the stabilizer component from the ceramic crystals. This loss undesirably results in gradual temperature-related re-alteration of the crystal structure over time, further leading to onset of the aforementioned cleaving and fracturing problems. Such stabilizer material loss and crystal alteration result in a corresponding ceramic degradation and reduction in life expectancy of the component, due to compromised performance properties.
For many applications, it has been learned that certain stabilizers are more volatile and susceptible to progressive high temperature loss than other stabilizers. Consequently, the more volatile stabilizers are frequently less desirable than the more loss-resistant stabilizers. For example, it has been learned that calcia (CaO) and magnesia (MgO) stabilizers are capable of providing a stabilized ceramic that initially achieves many of the desirable performance properties, but over time calcia and magnesia stabilizers may be more susceptible to loss than other less volatile stabilizers.
Still another problem particular to hydrocarbon pyrolysis pertains to carbon infiltration and coking within the porosity of the ceramic component, which at high temperature can produce a carbide-oxide conversion chemistry on the zirconia oxide that also leads to progressive component degradation, herein considered a type of “ceramic corrosion.” This newly recognized corrosion mechanism was not previously identified, understood, or recognized as a concern with high temperature hydrocarbon pyrolysis using ceramics.
The pyrolysis art needs a stabilized ceramic composition that provides the desirable set of performance properties and that can sustain those properties for a commercially meaningful period of use, by resisting loss of stabilizer, maintaining crystalline stability, and enduring prolonged exposure to high severity temperatures, substantial temperature swing cycles, cyclic flows of combustion and reaction materials. The desired materials must also resist the carbide-oxide corrosion problems. Still further, the desired materials must concurrently provide and maintain the needed structural integrity, crystalline stability, relatively high heat transfer capability, and chemical inertness required for large scale, commercial, high productivity applications, particularly those pertaining to hydrocarbon pyrolysis. Unavailability of such materials, components, and associated processes has been one of the most critical impediments to large scale, commercial adoption and application of many high temperature pyrolysis and chemistry processes and apparatus.